Direct thermal printing is a recognized means of printing quietly without toners or inks. It is a relatively mature technology that has been around for over forty years. Its use by retailers for printing of cash register receipts, mailing labels, etc. is now commonplace.
An example of early one-sided direct thermal printing is the thermal half-select printing as taught in U.S. Pat. Nos. 3,466,423 and 3,518,406 to John L. Janning. Such thermal half-select printing was accomplished by energization of electrically resistive thermal printing elements on both sides of thermal printing paper at the same time. The dual-sided coincident electrical current energization energy is additive to produce one-sided printing. The applied energy levels were such that, if applied on one side only, they were not sufficient enough to cause printing. By applying sufficient heat on both sides of the media simultaneously, the applied energies added and one-sided printing could occur.
Duplex or dual-sided direct thermal printing of transaction documents or receipts is described in U.S. Pat. Nos. 6,784,906 and 6,759,366. The printers were configured to allow printing on both sides of thermal media moving along a feed path through the printer. In such printers a direct thermal print head was disposed on each side of the media feed path. A print head faced an opposing platen across the feed path from the print head.
In direct thermal printing, a print head selectively applies heat to paper or other sheet media comprising a substrate with a thermally sensitive coating. The coating changes color when heat is applied, by which “printing” is provided on the coated substrate. For dual-sided direct thermal printing, the sheet media substrate may be coated on both sides.
Duplex or dual-sided direct thermal printing has been described for providing variable information on both sides of a paper receipt, e.g., to save materials and to provide flexibility in providing information to customers. The printing could be driven electronically or by computer using a computer application program which directs dual-sided printing.
Duplex or dual-sided direct thermal printing as described in U.S. Pat. Nos. 6,784,906 and 6,759,366 involves direct thermal print heads offset from one another while disposed on opposite sides of the media feed path for single-pass, two-sided printing. Unless there is a print head offset, uneven print density can potentially occur. This is because heat energy can be additive if it is applied simultaneously to both sides of the thermal printing paper when the print heads are directly across from one another.